Showing papers by "Gerry Gilmore published in 2003"

ISOGAL : A deep survey of the obscured inner Milky Way with ISO at 7 mu m and 15 mu m and with DENIS in the near-infrared

Abstract: The ISOGAL project is an infrared survey of specific regions sampling the Galactic Plane selected to provide information on Galactic structure, stellar populations, stellar mass-loss and the recent star formation history of the inner disk and Bulge of the Galaxy. ISOGAL combines 7 and 15 μ m ISOCAM observations – with a resolution of 6″ at worst – with DENIS IJK s data to determine the nature of the sources and the interstellar extinction. We have observed about 16 square degrees with a sensitivity approaching 10–20 mJy, detecting ~105 sources, mostly AGB stars, red giants and young stars. The main features of the ISOGAL survey and the observations are summarized in this paper, together with a brief discussion of data processing and quality. The primary ISOGAL products are described briefly (a full desciption is given in Schuller et al. 2003): viz. the images and the ISOGAL–DENIS five-wavelength point source catalogue. The main scientific results already derived or in progress are summarized. These include astrometrically calibrated 7 and 15 μ m images, determining structures of resolved sources; identification and properties of interstellar dark clouds; quantification of the infrared extinction law and source dereddening; analysis of red giant and (especially) AGB stellar populations in the central Bulge, determining luminosity, presence of circumstellar dust and mass-loss rate, and source classification, supplemented in some cases by ISO/CVF spectroscopy; detection of young stellar objects of diverse types, especially in the inner Bulge with information about the present and recent star formation rate; identification of foreground sources with mid-IR excess. These results are the subject of about 25 refereed papers published or in preparation.

ISOGAL: A deep survey of the obscured inner Milky Way with ISO at 7 and 15 micron and with DENIS in the near-infrared

Abstract: The ISOGAL project is an infrared survey of specific regions sampling the Galactic Plane selected to provide information on Galactic structure,stellar populations,stellar mass-loss and the recent star formation history of the inner disk and Bulge of the Galaxy. ISOGAL combines 7 and 15 micron ISOCAM observations - with a resolution of 6'' at worst - with DENIS IJKs data to determine the nature of the sources and theinterstellar extinction. We have observed about 16 square degrees with a sensitivity approaching 10-20mJy, detecting ~10^5 sources,mostly AGB stars,red giants and young stars. The main features of the ISOGAL survey and the observations are summarized in this paper,together with a brief discussion of data processing and quality. The primary ISOGAL products are described briefly (a full description is given in Schuller et al. 2003, astro-ph/0304309): viz. the images and theISOGAL-DENIS five-wavelength point source catalogue. The main scientific results already derived or in progress are summarized. These include astrometrically calibrated 7 and 15um images,determining structures of resolved sources; identification and properties of interstellar dark clouds; quantification of the infrared extinction law and source dereddening; analysis of red giant and (especially) AGB stellar populations in the central Bulge,determining luminosity,presence of circumstellar dust and mass--loss rate,and source classification,supplemented in some cases by ISO/CVF spectroscopy; detection of young stellar objects of diverse types,especially in the inner Bulge with information about the present and recent star formation rate; identification of foreground sources with mid-IR excess. These results are the subject of about 25 refereed papers published or in preparation.

Core radius evolution of star clusters

Abstract: We use N-body simulations of star clusters to investigate the possible dynamical origins of the observed spread in core radius among intermediate-age and old star clusters in the Large Magellanic Cloud (LMC). Two effects are considered, a time-varying external tidal field and variations in primordial hard binary fraction. Simulations of clusters orbiting a point-mass galaxy show similar core radius evolution for clusters on both circular and elliptical orbits and we therefore conclude that the tidal field of the LMC has not yet significantly influenced the evolution of the intermediate-age clusters. The presence of large numbers of hard primordial binaries in a cluster leads to core radius expansion; however, the magnitude of the effect is insufficient to explain the observations. Furthermore, the range of binary fractions required to produce significant core radius growth is inconsistent with the observational evidence that all the LMC clusters have similar stellar luminosity functions.

A charge-coupled device study of high-latitude Galactic structure: testing the model parameters

Abstract: We interpret published charge-coupled device (CCD) UBVI data to deduce the stellar density distribution and metallicity distribution function in the region of 2-8 kpc from the Galactic plane, and compare our results to several star-count models. A feature of extant star-count models is degeneracy between the adopted scaleheights of the thin and thick discs, and their local normalization. We illustrate the utility of this small data set, and future larger sets (e.g. Sloan Digital Sky Survey, SDSS), by explicitly considering consistency between the derived density laws, and the implied solar neighbourhood luminosity function. Our data set, from Hall et al.'s 1996 paper (I = 52°, b = -39°), contains 566 stars, selected to be consistent with stellar loci in colour-colour diagrams. The effective apparent V-magnitude interval is 15.5 ≤ V o ≤ 20.5. Our analysis supports the parametrization of the recent (SDSS) galaxy model of Chen et al., except in preferring the stellar halo axial ratio to be η = 0.84. Photometric metal abundances have been derived for 329 stars with (B - V) 0 ≤ 1.0 using a new calibration. This shows a multimodal distribution with peaks at [Fe/H] = -0.10, -0.70 and -1.50 and a tail down to -2.75 dex. The vertical distance-dependent metallicity distribution function, if parametrized by a single mean value, can be described by a metallicity gradient d[Fe/H]/dz ∼ -0.2 dex kpc - 1 for the thin disc and thick disc, and d[Fe/H]/dz ∼ -0.1 dex kpc - 1 for the inner halo, to z = 8 kpc. However, the data are better described as the sum of three discrete distribution functions, each of which has a small or zero internal gradient. The changing mix of thin disc, thick disc and halo populations with distance from the plane generates an illusion of a smooth gradient.

Chemical evolution of the M82 B fossil starburst

Abstract: M82 B is an old starburst site located in the eastern part of the M82 disc. We derive the distributions of age and metallicity of the star clusters located in this region of M82 by using theoretical evolutionary population synthesis models. Our analysis is based on the comparison of the BVIJ photometry obtained by de Grijs, O'Connel & Gallagher with the colours of single-generation stellar populations. We show that M82 B went through a chemical enrichment phase up to super-solar metallicities around the time of the last close encounter between M82 and its large neighbour galaxy M81. We date and confirm the event that triggered the enhanced cluster formation about 1 Gyr ago. At almost the same time an additional, distinct subpopulation of metal-poor clusters formed in the part of M82 B nearest to the galactic centre. The formation of these peculiar clusters may be related to infall of circumgalactic gas on to M82 B.

Can supermassive black holes alter cold dark matter cusps through accretion

Abstract: We present some simple models to determine whether or not the accretion of cold dark matter by supermassive black holes is astrophysically important. Contrary to some claims in the literature, we show that supermassive black holes cannot significantly alter a power-law density cusp via accretion, whether during mergers or in the steady state.

The analysis of isochrone fitting methods for red giant branch photometry, and tip red giant branch distance determination

Abstract: We consider the procedure of isochrone fitting and its application to the study of red giant branch (RGB) photometry in old stellar populations. This is extended to consider the problems introduced by the inclusion of lower-magnitude regions of the colour-magnitude diagram. We refer especially to our previous paper, where the details of our isochrone interpolation and fitting code are explained. We address the systematic errors inherent in the process of isochrone fitting, and we investigate the extent to which simple stellar populations can be recovered from noisy photometric data. We investigate the effects caused by inaccurate distance estimates, isochrone model variation and photometric errors. We present results from two studies of approximately coeval stellar populations, those of the Milky Way globular cluster system, and the Ursa Minor dwarf spheroidal galaxy. In addition, we introduce a new method for estimating distances using photometry of the tip of the RGB, which is significantly more robust than the standard edge-detection filter.

Characterising stellar micro-variability for planetary transit searches

Abstract: (abridged) A method for simulating light curves containing stellar micro-variability for a range of spectral types and ages, based on parameter-by-parameter scaling of a multi-component fit to the solar irradiance power spectrum (from VIRGO/PMO6 data), and scaling laws derived from ground based observations of various stellar samples, is presented. We observe a correlation in the Sun between the amplitude of the power spectrum on weeks timescales and the BBSO Ca II K-line index of chromospheric activity. On the basis of this evidence, the chromospheric activity level, predicted from rotation period and B-V colour estimates according to the relationship first introduced by Noyes(1983) and Noyes et al. (1984), is used to predict the variability power on weeks time scale. The rotation period is estimated on the basis of a fit to the distribution of rotation period versus $B-V$ observed in the Hyades and the Skumanich (1972) spin-down law. The characteristic timescale of the variability is also scaled according to the rotation period. We use this model to estimate the impact of the target star type and age on the detection capability of missions such as Eddington and Kepler. K stars are found to be the most promising targets, while the performance drops significantly for stars earlier than G and younger than 2.0 Gyr. Simulations also show that \emph{Eddington} should detect terrestrial planets orbiting solar-age stars in most of the habitable zone for G2 types and all of it for K0 and K5 types.

Mass Segregation in Young LMC Clusters

Abstract: We present the detailed analysis of Hubble Space Telescope observations of the spatial distributions of different stellar species in two young compact star clusters in the Large Magellanic Cloud (LMC), NGC 1805 and NGC 1818. Based on a comparison of the characteristic relaxation times in their cores and at their half-mass radii with the observed degree of mass segregation, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805. Both clusters were likely formed with very similar initial mass functions (IMFs). In fact, we provide strong support for the universality of the IMF in LMC clusters for stellar masses m >= 0.8 M_sun.